This invention relates to circuit breakers and particularly to circuit breaker trip coils and trip alarm circuits.
Conventionally, circuit breaker trip coils are energized from a battery or other DC source. Each trip coil has in series a current auxiliary relay and a plurality of tripping contacts. The tripping contacts formerly were electromechanical devices but with the development and popularity of solid state devices, these electromechanical tripping contacts have now been supplanted by solid state devices, particularly silicon controlled rectifiers (SCR).
The current auxiliary relay functions to actuate and seal in the breaker trip alarm circuit when the breaker is tripped. The current auxiliary relay must have low impedance and adequate sensitivity and must also have a substantial time delay to ensure that the alarm circuit locks in before the relay is automatically de-energized.
Conventional current auxiliary relays, one example of which is the General Electric Company HGA relay formerly in general use for actuating trip alarms, are unsatisfactory when the circuit includes multiple solid state tripping contacts such as SCR's, because the HGA relay is unable to withstand, without contact closure, the inrush current of the substantial capacitance required for the paralleled SCR tripping contacts. This substantial capacitance is needed for supplying transient energy surge suppression and for slowing down the wave front to prevent operation of the SCR's during energizing of circuit. With the HGA relays, the charging of these capacitors resulted in incorrect tripping indications because they could not be adjusted for both adequate insensitivity and required time delay. The maximum time delay capability of the HGA relay is about 70 milliseconds (effectively 40 milliseconds because of contact bounce) and to obtain this much delay, a very close setting of the contacts is required. This increases the sensitivity and aggravates the probability of false trip indication due to capacitor inrush current. Also, because of the inadequate time delay of the HGA relay, it cannot be used for automatic circuit breaker reclosing.